Mobile telephone system operators provide telephone service to subscribers by providing a mobile communications infrastructure in a geographic region or by negotiating access for subscribers to other mobile telephone system operators. Each mobile telephone system generally includes a plurality of cell sites distributed over a geographic region. Each cell site includes a base station having a transmitter and receiver for communicating with mobile telephones within a coverage area or “cell” around the cell site. Cell sites are placed such that there is a continuous or nearly continuous coverage area over a geographic region. This allows mobile telephone users the freedom to use a telephone over a wide geographic area.
The freedom of movement afforded by mobile telephones is advantageous. However, during an emergency such as a 911 telephone call originated from a mobile telephone, locating the mobile telephone is difficult as compared with locating a land-line originated 911 telephone call. This is because land line 911 calls may be located and correlated with a street address based on the telephone number of the land-line telephone. By contrast, the telephone number of a mobile telephone does not help to locate the mobile telephone because mobile telephone calls can be originated from anywhere within the service area.
One reason for the popularity of mobile telephones is that they can be used for emergencies when outside of the home. For example, a motorist could use the mobile telephone to call for assistance in case of a disabled vehicle. Many areas provide a special emergency mobile telephone number. In other areas, users can dial 911, just as they would from a conventional land-line telephone.
One of the problems with using a mobile telephone for emergencies stems from its mobility. Public service providers, such as police, may not know the location of a mobile telephone calling the emergency number. In addition, users who call the emergency number often cannot provide enough location information to allow the public service provider to find them. Thus, it is desirable to provide a mobile telephone system which can determine the location of a mobile telephone that calls an emergency number and to provide location information to public service providers, for example the police. A mobile telephone location service would also be desirable for providing location information to telephone users who are lost within the service area and to fleet vehicle operations who benefit from accurately knowing the location of fleet vehicles.
In general there are two approaches for determining the location of a mobile telephone. The first is to implement location technology in the mobile telephone itself. For example, a global position system (GPS) device may be included in a mobile telephone for determining a precise location. This location may then be transmitted to the mobile telephone network for use in locating the mobile telephone user. However, this approach is expensive to implement, may tend to enlarge mobile telephones and may be ineffective indoors and wherever the GPS satellite signals are blocked from reception by the mobile telephone.
The second approach is to design the location technology into the mobile telephone network without substantially modifying the mobile telephone. Using this approach, the location of a mobile telephone is determined based on an analysis of an incoming signal at one or more base stations in a mobile telephone network. The accuracy of the location determination may be improved by the reception and analysis of the incident signal from the mobile telephone at more than one base station. Receipt at three base stations, for example, affords the ability to determine location based on triangulation.
Mobile telephones generally are programmed to adjust transmission power so that a single base station clearly receives signals from the mobile telephone and the remaining base stations in the network receive the signals at a sufficiently low power level to not interfere with other mobile telephone transmissions. This approach, taken to improve quality and/or frequency reuse within a geographic region, adversely impacts the ability of a mobile telephone network to determine the location of a mobile telephone.
There is a need for a new method for determining the location of a mobile telephone which does not require costly modifications of the mobile telephone and which permits the accurate determination of the location of a mobile telephone. There is a further need for the method to be flexibly invoked, including in response to an emergency telephone call. The method should also determine location accurately enough to comply with the E911 regulations adopted by the Federal Communications Commission (FCC) in 1996.